How to use ODE45 to solve a second order system of equations?

Question

Mr_Turnip_Head 2016-2-9

回答： Torsten 2016-2-9

I need to solve:

d^2t/ds^2 = 0

d^2r/ds^2 = 0

I know that this is a straight line, however I'm going to extrapolate this to another example.

So what I have is

function dy = myode(t,y)
dy = zeros(2,1);
dy(1) = y(2);
dy(2) = 0;

I can call this buy using

[T,Y] = ode45(@myode,0:5,[1;0])

Any help is appreciated!

Edit:

 yin=[0 4];  % initial values
time=[0 5];
[x,y]=ode45(@myode,time,yin);
plot(x,y)

where myode is

 function dy=myode2(~,y)
 dy=[0;0];
 dy(1)=y(2);
 dy(2)= 0;

Now the problem is the plot. I get the correct straight line(blue) but what is this pesky red line at y = 4?

Answer 1

Torsten 2016-2-9

It's the derivative of y (y(2)) which is identically 4.

If you only want to plot y, use

plot(x,y(:,1));

Best wishes

Torsten.